1. Field of the Invention
This invention relates to a process for removing calcium oxalate scale from metal surfaces by contacting it with a mixture of nitric acid and manganese dioxide.
2. Description of the Prior Art
Calcium oxalate is a common substituent of scale forming on the metal surfaces of apparatus used for thermal treatment of aqueous solutions and suspensions. It forms most frequently in the tubes of heat exchangers. In the past, calcium oxalate scale has been removed by washing the pipes or tubes with dilute aqueous acid such as hydrochloric acid or nitric acid. However, the calcium oxalate has a very limited solubility in these acids so that repeated washings are necessary, involving a costly and time-consuming operation.
German Pat. No. 568,532, published Jan. 20, 1933, discloses a process for removing "Bierstein", a scale consisting in part of calcium oxalate from aluminum vessels used in the brewery industry, by treating the vessels with nitric acid and an oxidizing agent. The oxidizing agents disclosed are sodium peroxide, chromium trioxide, chromic acid, dichromic acid, chloric acid and permanganic acid salts.